It is known to those of skill in the art that a calender or calender stack is a series of rolls, usually steel or cast iron, mounted horizontally and/or stacked vertically. During machine calendering in a paper processing application, the dry paper passes between the rolls under pressure, thereby improving the surface smoothness of the paper caused by, for example, imperfections in felt marks, cockle lumps, fibrils, and the like. Additionally, such a calender stack can improve the gloss and create a more uniform caliper and porosity. These improvements can make the paper better suited for printing and decrease manufacturing problems during printing and rewinding operations. As would be known to those of skill in the art, a typical loading range between opposed rolls generally varies from 0 N/cm (Gap) to 85,000 N/cm (0 lbs. per linear inch (Gap)-1,000 lbs. per linear inch).
Some known calendering systems are provided with a steel roll and a roll having a rubberized coating. In such systems, the steel roll is known as the king roll and it may be located in the top or bottom position of the calender. The king roll may be larger or smaller than the other rolls in the calender stack and may be crowned (i.e., has a larger or smaller diameter in the center of the roll as compared to the ends) in order to permit even pressure being applied to a substrate passing between opposing loaded roll faces. However, one of skill in the art will realize that the king roll and/or the queen roll can be crowned and/or provided with variable crown capability. A variable crown can be achieved using various methods including, a pressurized oil filled roll where the oil pressure controls the degree of crowning, internal hydraulic shoes that press against the roll shell to control the degree of crowning, or roll bending. The roll in mateable engagement with the king roll is known as the queen roll. In certain operations, the queen roll can be provided with a rubberized coating in order to increase the engagement of the surface of the queen roll with the surface of the king roll.
In conventional calendering operations, as the two rolls come in contact, one or both surfaces of the king roll and/or queen roll deform. In operations where the queen roll is provided with a rubberized coating, such a coating will be provided on the queen roll in about ½-inch to 1-inch (1.27 cm to 2.54 cm) in thickness. As the surface of the rubberized queen roll deforms, the rubberized coating deforms in order to pass through the nip formed between the king roll and queen roll. This cover flows to conform to the nip surface. Such conformation can result in shear forces being formed across the area of contact between the two rolls.
A second mechanism that can create shear forces across a nip in a calendering operation exists when one roll of the calender attempts to drive the second roll. As one roll attempts to speed up or slow down, it forces the rubberized coating deposited upon the second roll to deform in such a way as to force the second roll to speed up or slow down. In doing so, the interaction between the first and second rolls of the calender create a shear force that is transmitted through a substrate disposed therebetween. This shear force cannot be avoided in a calendering operation with only one driven roll. These forces can be generated by rolls of a calender system having steel rolls and/or rolls having no coating disposed thereon due to frictional forces caused by roll deformation.
When the rolls forming the calender nip are separately driven and are forced together, they are provided with the capability of transferring forces across the nip to drive each roll. If the rolls tend towards asynchronous behavior (i.e., the rolls are not surface speed matched in the nip), a net torque is developed between the rolls with associated forces across the nip, and the resulting calendering operations can become unpredictable. The nip torque imbalance creates a shear force across a material passing between the rolls of the nip that is greater than the shear forces caused by the roll deformation alone. This shear force can damage a substrate placed between the rolls of a calender system.
A known method for controlling the shear force developed across the nip in a calendering operation provides for an operator to manually set the torques between multiple drives to minimize the shear force transmitted through the substrate. The most common means to manually manipulate the torque division between the multiple drives are 1) through torque division to multiple motors of a common speed controller output, 2) operating one drive to control speed and one to provide a constant torque or 3) operating one speed controller as a lead, or master, speed controller and the second as a droop, or current compounded, speed controller.
Such systems may be suitable for use in situations where constant loading of the rolls of a calender system is utilized. However, some processes require variable calender loading as the product (such as paper) passes between the calender rolls. In variable calender loading systems where total motor torque loads can change, manual adjustments such as those used in constant loading processes, are not suitable. This is because an operator of a variable calender system would be required to provide continual (if not continuous) adjustments to the motor torques to maintain the desired minimum level of shear force in the nip.
Thus, it would be useful to provide for a method to control torque in a calendering system that keeps one roll torque (or current) at a desired value while a second roll (preferably rubber covered) is nipped against the first roll. Such a mechanism would effectively change the torque on the second roll to affect a change of the torque utilized by the first roll. Such a process would control the amount of shear forces developed across a substrate passing between the calender rolls. This can minimize the shear damage to the substrate and improve the tensile loss during a calender, combiner, or embosser/laminator operation. This can effectively reduce web losses through reduced substrate damage by minimizing shear forces transmitted across the substrate.